High capacity magnetic recording devices using rigid or flexible disks as the recording medium require servo information for the accessing and following of data tracks. This servo information may be recorded on a separate disk surface dedicated to servo information; or interleaved with data records on the same disk surface; or even as a combination of both systems. The present invention is particularly concerned with recording devices in which the servo information is interleaved with data on the same record medium. One such arrangement is described in our published UK patent specification No. 1499268. In this publication, a record device is described in which servo position reference information is pre-recorded in a plurality of spaced servo records interleaved between larger regions reserved for data. The nominal data track center-line of each data region in a sector is defined by the servo information held in the immediately preceding servo record in that sector. Since, in this system, the servo information is contiguous with the subsequently recorded data, the data tracks of interest can be followed with greater accuracy and thus packed closer together. During use, servo records are sampled at times defined by clock signals, and servo position error signals produced by demodulating the servo signals derived by a recording head (transducer) reading the pre-recorded servo information are employed in a closed servo loop to drive the transducer positioning mechanism to reduce any positional off-set and thus to maintain the transducer in the desired on-track position.
The pre-recorded servo information on the record medium is often in the form of recorded signal blocks provided on each side of a pre-defined boundary line. This line is by definition co-linear with the center-line of the succeeding data track in the same sector, and a pre-ceding data track in the preceding sector. It will be appreciated that the precise nature of the pre-recorded servo information has little bearing on the present invention and many alternative encoding schemes are available. One suitable method of encoding servo information for example is described in an IBM Technical Disclosure Bulletin article entitle "Null servo pattern" by A. J. Betts, Vol. 18, No. 8, January 1976, pages 2656/7. Generally speaking, the sampled servo position error signal produced by demodulating the transducer signal derived from a servo record in a sector is in the form of an electrical signal, the magnitude and polarity of which indicates the amount and direction of off-set of the transducer reading the sample from the defined servo boundary, and thus defines the nominal data track position for that sector. The error signal derived from the servo sample may be maintained in analogue form for continuous network compensation in an analogue closed servo loop or converted into digital form and supplied to a digital controller for signal manipulation before being reconverted to analogue form to close the servo loop.
The mathematics, design and analysis of the operation of sample servo systems using either analogue or digital techniques is well known. Further information on such systems may be obtained by reference for example to the text book "Sampled-data control systems" by J. R. Ragazzini and G. F. Franklin, published by McGraw-Hill Book Company Inc. 1958. Chapter 7 of this book is of particular interest in that it is concerned with digital compensation of sampled data systems.
The recent reduction in cost of digital processing occasioned by the advent of large-scale integration of digital circuits together with the development of the microprocessor as a standard article has made digital processing more cost effective and the use of digital circuits in servo control loops is now becoming more wide-spread. As will become apparent, the present invention is applicable to servo positioning systems which are fully analogue in nature, as well as to systems in which conversion of the position error signal to digital form takes place as required by so-called digital servo control systems. In view of the reduction of cost of digital processing together with the advantage of avoiding the need for the precision components of analogue systems, a digital servo control system has been adopted to implement the invention and will be described hereinafter as the preferred embodiment of the invention.